Field of the Invention
The present invention relates to an optical system and an optical apparatus.
Description of the Related Art
In general, the optical performance of the image pickup optical system is evaluated based on the imaging performance of a focused object. However, depending on applications, an appearance of a defocused image may be an important evaluation factor for the optical performance of the image pickup optical system. In particular, in an image pickup optical system used for an image pickup apparatus, such as a film-based camera, a digital camera using a semiconductor image sensor, a video camera, and a TV camera, it is likely that the appearance of the defocused image is regarded as more important.
Japanese Patent Laid-Open No. (“JP”) 9-236740 proposes an image pickup lens system that includes an apodization (“APD”) filter located near a diaphragm. This lens system changes a length in at least one lens unit, and entirely moves to the object side in focusing from an infinity in-focus state to a short-distance in-focus state.
In general, in image pickup optical systems from a wide angel end to an intermediate telephoto end, the sagittal halo of an off-axis light flux causes an uneven intensity of the defocused image in the image periphery, and the ADP filter is effective to removing the sagittal halo. However, the APD filter disclosed in JP 9-236740 is configured so that its transmittance distribution gradually decreases as a distance from the optical axis increases in a direction perpendicular to the optical axis and forms an approximately Gaussian distribution. Therefore, the transmission of light near the light flux center other than the sagittal halo also decreases. As a result, a light amount drops disadvantageously.